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Cowdery, T.K. 1997, Shallow ground-water quality beneath cropland in the
Red River of the North Basin, Minnesota and North Dakota, 1993-95. U.S.
Geological Survey Water-Resources Investigations Report 96-4001 52 p.
Abstract
During 1993-95, the agriculture on two sandy, surficial aquifers in the Red
River of the North Basin affected the quality of shallow ground water in each
aquifer differently. The Sheyenne Delta aquifer, in the western part of the
basin, had land-use, hydrogeological, and rainfall characteristics that
allowed few agricultural chemicals to reach or remain in the shallow ground
water. The Otter Tail outwash aquifer, in the eastern part of the basin, had
characteristics that caused significant amounts of nutrients and pesticides to
reach and remain in the shallow ground water. Shallow ground water from both
aquifers is dominated by calcium, magnesium, and bicarbonate ions. During the
respective sampling periods, water from the Sheyenne Delta aquifer was mostly
anoxic and water from the Otter Tail outwash aquifer had a median dissolved
oxygen concentration of 3.6 mg/L (milligrams per liter). The median nitrate
concentration was 0.03 mg/L as nitrogen (mg/L-N) in shallow ground water from
the Sheyenne Delta aquifer and 6.1 mg/L-N in that from the Otter Tail outwash
aquifer. Of 18 herbicides and 4 insecticides commonly used in the aquifer
areas and for which analyses were done, 5 herbicides and 1 herbicide
metabolite were detected in the shallow ground water from the Sheyenne Delta
aquifer and 8 herbicides and 2 metabolites were detected in that from the
Otter Tail outwash aquifer. The total herbicide concentration median was less
than the detection limit in shallow ground water from the Sheyenne Delta
aquifer and 0.023 µg/L (micorgrams per liter) in that from the Otter Tail
outwash aquifer. Triazine herbicides were the most commonly detected
herbicides and were detected at the highest concentrations in the shallow
ground water from both study areas. One sample from the Sheyenne Delta
aquifer contained a high concentration of picloram. Agricultural chemicals in
both aquifers were stratified vertically and their concentration correlated
inversely with ground-water age. The highest concentrations of these
chemicals and the youngest ground-water ages were at the water
table. Concentrations decreased and age increased with water-table depth.
Nitrate concentration varied seasonally over one-half an order of magnitude,
though concentrations only repeated seasonally in some shallow ground water.
Land-use factors that increased nitrate and herbicide concentrations were
greater tilled area, chemical application, irrigation, and cropland
contiguity. Hydrogeological factors that increased these concentrations were
a deeper water- table (higher oxygen concentration and less organic carbon),
larger grain-size and degree of sorting of aquifer material (shorter time in
the soil zone and aquifer), and fewer sulfur-containing minerals (lignite and
pyrite) composing the aquifer. High rainfall, just before sampling of the
Sheyenne Delta aquifer, contributed to the relatively low nitrate and
pesticide concentrations in the shallow ground water of this aquifer by
raising the water table higher into the soil zone, increasing ponded water
(increasing biodegradation), preventing some chemical application (flooded
fields), and leaching and then displacing nitrate-rich water downward, beneath
new recharge. The shallow ground-water quality measured beneath cropland in
these land-use study areas covers a large range. The land-use,
hydrogeological, and rainfall factors controlling this quality also control
shallow ground-water quality in other surficial aquifers in the Red River of
the North Basin. Although not used for drinking water, 43% of the shallow
ground water from the Otter Tail outwash aquifer was above the
U.S. Environmental Protection Agency\xd5 s nitrate maximum contaminant level
of 10 mg/L-N, reducing its potential uses. These high nitrate concentrations
do not threaten the Otter Tail outwash aquifer\xd5 s surface-water bodies with
eutrophication however, because significant denitrification occurs beneath
riparian wetlands before ground water discharges to surface waters.
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U.S. Geological Survey